Abstract:
The Atlantis Massif is an oceanic core complex located at the Mid-Atlantic Ridge at ca. 30 degrees N. During IODP Leg 304/305 Hole U1309D the footwall of the detachment fault, which is related to exhumation of the Massif, was drilled. It consists mainly of gabbros and troctolites, with minor amounts of basaltic and ultramafic rocks. The 1416 m long drilled section is fractured and shows a retrograde overprint recorded by granulite- to zeolite-facies mineral assemblages. Late-stage mineral veins (consisting of anhydrite, calcite, prehnite or zeolite) formed from sub-seafloor fluid-rock interactions. These veins were examined to further our understanding of the fluid regime and temperature conditions in detachment fault systems. Abundant syn- to postkinematic calcite has low concentrations of incompatible elements (e.g. U, Sr, Li) as well as flat chondrite-normalized REE+Y pattern with a positive Eu anomaly. This indicates that the calcite precipitating fluids are similar to basalt-hosted hight-T vents and indicate no affinity to the nearby serpentinization-derived Lost City vent field. The deep origin of the fluids is highlighted by low (super 87) Sr/ (super 86) Sr (0.704 to 0.708), mantle-like delta (super 7) Li (sub LSVEC) (+0.8 to +9.4 ppm) and delta (super 13) C (sub PDB) (-6 to -2 ppm). From delta (super 18) O values, minimum calcite precipitation temperatures of 150-220 degrees C are derived. Anhydrite and anhydrite + zeolite veins have (super 87) Sr/ (super 86) Sr values consistent with anhydrite formation from down-flowing seawater which had leached only minor amounts of Sr from the basement. The REE pattern of anhydrite veins indicate that admixed hydrothermal fluids at depth played a minor role. Silicate minerals (prehnite, quartz, plagioclase) predominate veins in the deepest section of Hole 1309D and indicate precipitation temperatures ranging from 270 to 145 degrees C (estimated from delta (super 18) O values). They are comparatively unradiogenic in (super 87) Sr/ (super 86) Sr (0.7033-0.7046) and demonstrate (in contrast to anhydrite) enhanced intensity of reactions between infiltration seawater and basement with increasing depth.